Method for preventing the formation of rust on the surface of coiled steel strip and an inhibitor for accomplishing same

ABSTRACT

Method for preventing the formation of rust on coiled steel comprising temper rolling annealed cold rolled steel strip, coating the steel strip with a liquid phase-vapor phase corrosion inhibitor and coiling the steel strip. The liquid phase-vapor phase inhibitor consists of an aqueous solution of a non-volatile nitrite of an alkali metal or an alkaline earth metal or volatile amine which is soluble in water and not subject to autodecomposition at ambient temperature. The volatile base compounds are, for example, ammonium hydroxide or ammonium carbonate. The nitrite radical is present in an amount of about 0.07% by weight up to the solubility of the nitrite compound. The base compound is about 0.1% by weight up to about 25% by weight of the aqueous solution. The aqueous solution has a pH of not less than about 8 to about 13.

United States Patent [1 1 Shoemaker 1 METHOD FOR PREVENTING THE FORMATION OF RUST ON THE SURFACE OF COILED STEEL STRIP AND AN INHIBITOR FOR ACCOMPLISHING SAME [75] lnventor: Carlyle E. Shoemaker, Bethlehem,

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: Apr. 1, 1971 [21] Appl. No.: 130,446

Related U.S. Application Data [63] Continuation of Ser. No. 830,120, June 3, 1969,

abandoned.

[52] U.S. Cl l48/6.I4 R, 117/49, 117/66 [51] Int. Cl. C231 7/00 [58] Field of Search 1 17/66, 49;

l48/6.14 R, 31.5, 18; 252/290; 72/46; 2l/2.7

[56] References Cited UNITED STATES PATENTS 1,676,685 7/1928 Cammen 72/46 1,933,188 10/1933 Sargent... l48/3l.5 1,982,065 11/1934 Oresco.... 72/46 2,243,853 6/1941 Castner l48/31.5 2,442,485 7/1948 Cook 117/50 2,550,997 5/1957 Harden et a1... 14816.15 3,421,925 l/1969 Hair et al 117/66 [451 Dec. 18, 1973 2,820,016 l/l958 Raifsnider et a1 252/290 OTHER PUBLlCATlONS Larke, Eustace C., The Rolling of Strip Sheet & Plate, London, Chapman and Hall Ltd., 1957. p. 370. TS340L3 Primary Examiner-William D. Martin Assistant Examiner-Janyce A. Bell Attorney-Joseph J. QKeefe [57] ABSTRACT auto-decomposition at ambient temperature. The volatile base compounds are, for example, ammonium hydroxide or ammonium carbonate. The nitrite radical is present in an amount of about 0.07% by weight up to the solubility of the nitrite compound. The base compound is about 0.1% by weight up to about 25% by weight of the aqueous solution. The aqueous solution has a pH of not less than about 8 to about l3.

15 Claims, No Drawings METHOD FOR PREVENTING THE FORMATION OF RUST ON THE SURFACE OF COILED STEEL STRIP AND AN INHIBITOR FOR ACCOMPLISHING SAME Cross-Reference to Related Application This is a continuing application of my original application Ser. No. 830,120 filed June 3, 1969, and now abandoned, entitled Rust Inhibitor Composition.

BACKGROUND OF THE INVENTION This invention in general relates to a method for preventing the formation of rust on ferrous metal and more specifically to the prevention of rust on the surface of cold rolled and annealed coiled steel strip and to the liquid phase-vapor phase inhibitor used to prevent the formation of rust.

Cold rolled and annealed steel strip is temper rolled to obtain the desired flatness and size and to impart an amount of stiffness to the steel. The temper rolled steel strip is formed into tightly coiled form for shipment or storage prior to shipment. The coiled steel is stored in large warehouses for varying lengths of time. Due to varying humidity conditions under which the coils are stored or shipped and the presence of corrosive salt residues deposited on the coil surfaces from cooling water used in reduction operations, surface rusting, referred to as speckled rust, occurs between the tightly coiled layers of steel unless special precautions are taken. It is impractical to control the humidity in the storage bays and shipping facilities because of the large ton nages of coils produced on a cold reducing mill. Other means must be used to prevent the formation of the rust on the surfaces of the coils.

Steel will rust when exposed to the atmosphere. The susceptibility of steel to rust is doubled when the steel is in a tightly packed condition such as coils formed after temper rolling or cold rolling.

lt has been common practice in the prior art to apply mineral or slushing oil to the cold rolled and annealed strip by roller coating or spraying at the temper mill prior to recoiling. The oil presents a measure of protection against rusting, although oil tends to dewet on the strip, leaving dry areas which later rust from contact with moist air. Then, too, the oil can be replaced by moisture in the air, reducing the effectiveness of the oil. Furthermore, end uses of the strip nearly always require removal of the oil prior to fabrication. Oil is messy. It creates working hazards due to slippery floors when being applied or removed from the strip. The oil requires special solventsfor removal and is therefore a costly operation both to apply and to remove from the steel strip.

Prior art practices to prevent corrosion of ferrous articles include the use ofliquid phase or vapor phase inorganic and organic inhibitors, such as sodium nitrite, or amine nitrites such as dicyclohexylammonium nitrite, di-isopropylammonium nitrite and the like, as exemplified in U. S. Pat. No. 2,550,917 issued May I, l95l to Gail D. Harden et al. entitled lnhibition of Corrosion" and U. S. Pat. No. 2,643,!77 issued June 23, l953 to Aaron Wachter et al., respectively. U. S. Pat. No. 2,550,9l7 shows that aqueous solutions of nitrites and carbonates are useful in protecting ferrous articles which remain wet, for example, underwater pipes and petroleum storage tanks on tankers. However, when the articles dry, the inhibitors so used also dry and separate into individual compounds and lose their effectiveness. When the dried articles coated with the inhibitors become wet because of humid atmospheric conditions, the inhibitors are washed off the surface of the articles thereby exposing the surface of rust-producing agents, such as moisture and oxygen and sulfur dioxide in the atmosphere. Obviously, the inhibitors are useful for articles which remain wet but are not useful for articles which are intermittently wet and dry.

The use of vapor phase inhibitors, as shown in U. S. Pat. No. 2,643,177, while successful in preventing the rusting of ferrous articles covered with a wrapping, such as paper soaked in the inhibitor, has not proven successful in protecting large ferrous articles, such as coils of steel, since the vapor cannot penetrate between the layers of the tightly coiled steel. As noted previously, the rusting of steel occurs more rapidly on surfaces between two pieces of steel which are clamped together than on steel which has a surface exposed to the atmosphere. Since the vapor of the vapor phase inhibitor cannot penetrate between the layers of the coiled steel they do not protect the steel and rusting in the form of speckled rust occurs. Steel which is coated with speckled rust cannot be coated with vitritied enamel, paint or many other coatings. Hence, speckled rust causes the rejection of the coils of steel.

It is therefore an object of this invention to provide a method which prevents speckled rust from forming on coiled cold rolled annealed steel strip.

It is another object of this invention to provide a rust inhibitor which can be readily removed from the strip surface.

It is an object of this invention to provide a method for preventing the formation of rust on the surfaces of tightly coiled steel after temper rolling wherein the steel is coated with a liquid phase-vapor phase inhibitor prior to coiling.

Summary of the Invention l have found that the foregoing objects can be obtained by use of an aqueous solution of an inorganic nitrite or an organic nitrate and ammonium hydroxide or ammonium carbonate. The nitrites suitable for this invention include the alkali metal nitrites, alkaline earth metal nitrites and the amine nitrites which are not subject to auto-decomposition at ambient temperature. The solution is effective in inhibiting rust on coils of steel strip when the total solution components range anywhere from about 0.2% by weight up to saturation.

It is essential that the pH of the solution be controlled with certain prescribed limits, and there must be present sufficient ammonia to supply an alkaline vapor to volatilize at least a part of the solution.

The inhibitor of this invention, which is a liquid phase-vapor phase inhibitor, is particularly effective when used in conjunction with cold rolled strip in coiled form.

Detailed Description As one example of the inhibitor of this invention, 5 weight per cent sodium nitrite and 5 weight per cent ammonium hydroxide are dissolved in tap water to form a solution. The pH of the solution is l 1.6. This solution if kept in a closed container, will remain at approximately the sa me concentration indefinitely at ambient temperature. The inhibitor solution can be applied to annealed steel strip in the cold condition by spraying the strip prior to or as it leaves the temper mill rolls but at any rate prior to coiling. Inhibitor solution is sprayed across the entire width of the strip. The solution may be sprayed on the strip prior to entry of the strip into the rolls, but the application of the solution after the rolls is preferred. Spraying one side, preferably the underside, of the strip is sufficient, as both sides are made wet during subsequent coiling. After the strip is coiled, the wet inhibitor remains between the coil laps.

During storage and subsequent shipping, the inhibitor solution protects all portions of the strip, wet or dry, which it covers. In addition, at the concentrations specified, a small amount of ammonia is gradually released. As ammonia by itself will not prevent rusting of steel surfaces, it is believed that the ammonia that volatilizes, carries with it a small amount of nitrite, and that this combination of vapor protects any uncoated dry areas within the coil laps. Thus, the inhibitor acts as both a vapor phase and liquid phase inhibitor within the coil. In addition, once all water has been evaporated, leaving behind a dry residue, the effective ingredients of the inhibitors act in liquid phase in the immediate locale of the residue. Because of the slow release of ammonia in the coil at ordinary conditions of storage and transit, the inhibitor remains effective for extended storage periods, usually until the coil is unwound for further processing.

For protection of the steel over extended periods, the inhibitor solution should be applied so that the active components of the solution (sodium nitrite and ammo nium hydroxide in the above example) are present at the time of application preferably in an amount not less than approximately 3 micrograms per square centimeter of strip surface. For short period protection, lesser amounts of inhibitor may be applied with satisfactory results. Any amount of inhibitor solution above the Background of the Invention to the amount which can be retained physically on the coiled strip. In the foregoing example, the amount of active components of the inhibitor solution represents about 6 micrograms per square centimeter.

In another example of an inhibitor solution of this invention, and its use, 3 weight per cent of potassium nitrite and 3 weight per cent of ammonium carbonate are dissolved in water to produce a solution having a pH of 8.7. In this example, the cold rolled annealed strip, after leaving the temper mill, is coated with the inhibitor solution by immersing the strip in the solution contained in an open tank. After immersion coating, the strip is coiled, as in the first example. Ammonium carbonate, while less volatile than ammonium hydroxide, is nonetheless effective in protecting the coiled strip. Because of the lower volatility, inhibitor with ammonium carbonate can be expected to provide protection over a longer period. However, either ammonium hydroxide or ammonium carbonate in conjunction with nitrite will produce an inhibitor which will protect the coil for many weeks under normal conditions.

In one test, run according to the first example, using ammonium hydroxide with sodium nitrite, the treated coil was still wet after 29 weeks of mill storage. No rust appeared on the coiled strip surface, although there was slight rusting at .the strip edges exposed to the atmosphere.

Effective inorganic nitrites in this invention include any of the alkali metal nitrites and alkaline earth metal nitrites; however, sodium or potassium nitrite are generally preferred because of cost factors and availability.

Organic nitrites may also be used, including any nitrite salts of aromatic, aliphatic or heterocyclic amines which are not subject to auto-decomposition under ordinary conditions of use at ambient temperature. Examples of the types of nitrite salts of amines useful in this invention are the following:

Dicyclohexylammonium nitrite Diisopropylammonium nitrite 2,4,4,6-tetramethyltetrahydro-l, 3 oxazine nitrite ,B-phenylethylammonium nitrite Mono-cyclohexylammonium nitrite While any of the numerous organic nitrites in the above-described class of compounds are comprehended by this invention, preferance again favors use of the sodium or potassium nitrites, many organic nitrites being difficult to procure in commercial quantities, are sparingly soluble, unstable, or develop objectionable fumes.

As one example of the use of an organic nitrite in this invention, 5 weight per cent diisopropyl ammonium nitrite and 5 weight per cent ammonium carbonate are mixed with water to form a solution having a pH of 8.8. This solution is applied to the underside of a cold rolled annealed strip after it leaves the tandem mill and prior to coiling to produce a coating on both sides of the strip as the strip is wound, the wet inhibitor solution being retained within the laps of the coils.

A laboratory test was performed on wet panels to determine the improvement of inhibitor-treated panels over untreated panels.

In this test, four 4-inch by 6-inch panels taken from 20 gage cold rolled annealed strip were immersed in an aqueous solution of 5 weight per cent sodium nitrite and an aqueous solution of 5 weight per cent sodium nitrite and 5 weight per cent ammonium hydroxide, with 0.1% sodium chloride added to the solution to create a corrosive condition. The weight of the active ingredients of the solution applied to the panels was approximately 3.6 micrograms per square centimeter. The wet panels were placed together and secured as a tight pack, simulating tightly coiled strip.

Four untreated specimens, from the same strip source as the treated specimens, also in the form of 4- inch by 6-inch panels, were immersed in water and, while still wet, formed into a tightly secured pack with a glass cover to allow observation.

Both packs were placed in a humidity cabinet and subjected to the conditions shown in the following table:

Exposure Temperature Humidity days F, 2 room 40-45 3 85 2 room 60 4 85 85 3 room low After exposure, each pack was opened and inspected. Non-treated panels showed considerable rusting throughout the entire surface area of each panel. The treated panels showed no rust, indicating the unique possibilities of the inhibitor solution under the much less severe conditions encountered in the storage of steel packs or coils in commercial practice. I have found that even though the steel is more susceptible to rusting when in a tight pack than exposed to the atmosphere, the above tests showed that, the steel treated by the method of the invention was protected from rusting when wet or dry or in an alternate wet and dry condition. The problem of rusting of steel strip in a tight coil has been achieved.

The inhibitor solution is operable throughout a wide range of concentrations. The amount of nitrite radical may range from about 0.07% by weight, up to the solubility of the nitrite compound used. Preferably, the amount of nitrite radical is held between 0.7% and 7% of the aqueous solution. The base compound may be present in the solution in a range from about 0.1% to 25%, and preferably should range between 1% and 10%.

The ratio of nitrite to ammonium compound (total base), should be held within the range of from 0.1 :l to 10:1. For optimum performance the ratio should be held within the range of from 0.521 to 2:1.

Generally, it will be most convenient to make up the inhibitor using ammonium compound alone as the basic component. However, as long as ammonium compound is a principal component sufficient to supply the necessary volatility and to develop the proper pH,

a certain amount of other alkaline compound, such as sodium or calcium hydroxide, may be present as part of the basic component.

The pH of-the inhibitor solution should not drop below about 8 or rise above 13. A preferred pH range is from 8.5 to 12.

The amount of inhibitor solution applied to the strip is not critical. The solution can be applied in an amount of from 1 microgram of active ingredients (nitrite compound and base) per square centimeter of strip or sheet surface, up to any practical amount. A range of .from 3 to micrograms of active ingredients persquare centimeter would be preferred under practicallyall operating conditions. It may be necessary to vary the amount of inhibitor applied at times to insure a uniform coating at different strip speeds when the coating is ap* plied as the strip leaves the temper mill, where speeds up to 3000 feet per minute may be attained.

1 claim:

l. A method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising:

a. temper rolling steel strip,

b. coating the steel strip with a corrosion inhibitor 5 consisting essentially of an aqueous solution of at least one nitrite compound taken from the group consisting of alkali metal nitrites, alkaline earth metal nitrites, and amines not subject to autodecomposition at ambient temperature wherein a nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the nitrite compound and a base compound in which the principal portion of the base is at least one volatile compound taken from the group consisting of ammonium hydroxide and ammonium carbonate in which said base represents from about 0.1% to about by weight of said aqueous solution, said solution having a pH of not less than 8, and

c. coiling the steel'strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented.

2. The method of claim 1 in which the nitrite compound of step (b) is sodium nitrite.

3. The method of claim 1 in which the nitrite compound of step (b) is potassium nitrite.

4. The method of claim 1 in which the nitrite compound of step (b) is dissopropyl ammonium nitrite.

5. The method of claim 1 in which the base compound of step (b) is ammonium hydroxide.

'6. The method of claim 1 in which the case compound of step (b) is ammonium carbonate.

7. The method of claim 1 in which the aqueous solution of step (b) has a nitrite radical to base ratio of from about 0.07:] to about 10:1.

8. The method of claim 1 in which the nitrite radical of step (b) is present in the solution in the amount of from 0.7% up to the solubility of the nitrite compound and the base compound of step (b) is present in the solution in an amount of from 1% to 10% by weight.

9. The method of claim 1 in which only one side of the strip is coated with nitrite inhibitor solution.

10. The method of claim 1 in which both sides of the strip are coated with nitrite inhibitor solution.

11. The method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising:

a. temper rolling steel strip,

b. coating the steel strip with a corrosion inhibitor consisting essentially of an aqueous solution of sodium nitrite and ammonium hydroxide, wherein the nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the sodium nitrite and the ammonium hydroxide represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8, and

c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the'surfaces of the steel within the laps of the coils is prevented.

12. A method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising:

a. coating a side of steel strip with a corrosion inhibitor consisting essentially of an aqueous solution of at least one nitrite compound taken from the group consisting of alkali metal nitrites, alkaline earth metal nitrites, and amines not subject to autodecomposition at ambient temperature wherein a nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the nitrite compound and a base compound in which the principal portion of the base is at least one volatile compound taken from the group consisting of ammonium hydroxide and ammonium carbonate in which said base represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8,

b. temper rolling the steel strip under conditions in which the corrosion inhibitor solution is not dried,

and

c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented.

113. The method of claim 12 in which only one side ofthe steel strip is coated with nitrite inhibitor solution.

14. The method of claim 12 in which both sides of the strip are coated with nitrite inhibitor solution.

15. The method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising:

a. coating a side of steel strip with a corrosion inhibitor consisting essentially of an aqueous solution of sodium nitrite and ammonium hydroxide, wherein the nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the sodium nitrite and the ammonium hydroxide represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8,

b. temper rolling the steel strip under conditions in which the corrosion inhibitor solution is not dried, and

c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented. 

2. The method of claim 1 in which the nitrite compound of step (b) is sodium nitrite.
 3. The method of claim 1 in which the nitrite compound of step (b) is potassium nitrite.
 4. The method of claim 1 in which the nitrite compound of step (b) is dissopropyl ammonium nitrite.
 5. The method of claim 1 in which the base compound of step (b) is ammonium hydroxide.
 6. The method of claim 1 in which the case compound of step (b) is ammonium carbonate.
 7. The method of claim 1 in which the aqueous solution of step (b) has a nitrite radical to base ratio of from about 0.07:1 to about 10:1.
 8. The method of claim 1 in which the nitrite radical of step (b) is present in the solution in the amount of from 0.7% up to the solubility of the nitrite compound and the base compound of step (b) is present in the solution in an amount of from 1% to 10% by weight.
 9. The method of claim 1 in which only one side of the strip is coated with nitrite inhibitor solution.
 10. The method of claim 1 in which both sides of the strip are coated with nitrite inhibitor solution.
 11. The method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising: a. temper rolling steel strip, b. coating the steel strip with a cOrrosion inhibitor consisting essentially of an aqueous solution of sodium nitrite and ammonium hydroxide, wherein the nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the sodium nitrite and the ammonium hydroxide represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8, and c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented.
 12. A method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising: a. coating a side of steel strip with a corrosion inhibitor consisting essentially of an aqueous solution of at least one nitrite compound taken from the group consisting of alkali metal nitrites, alkaline earth metal nitrites, and amines not subject to auto-decomposition at ambient temperature wherein a nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the nitrite compound and a base compound in which the principal portion of the base is at least one volatile compound taken from the group consisting of ammonium hydroxide and ammonium carbonate in which said base represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8, b. temper rolling the steel strip under conditions in which the corrosion inhibitor solution is not dried, and c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented.
 13. The method of claim 12 in which only one side of the steel strip is coated with nitrite inhibitor solution.
 14. The method of claim 12 in which both sides of the strip are coated with nitrite inhibitor solution.
 15. The method for preventing the formation of speckled rust within the laps of cold rolled and annealed coils of steel, comprising: a. coating a side of steel strip with a corrosion inhibitor consisting essentially of an aqueous solution of sodium nitrite and ammonium hydroxide, wherein the nitrite radical is present in an amount of from about 0.07% by weight up to the solubility of the sodium nitrite and the ammonium hydroxide represents from about 0.1% to about 25% by weight of said aqueous solution, said solution having a pH of not less than 8, b. temper rolling the steel strip under conditions in which the corrosion inhibitor solution is not dried, and c. coiling the steel strip into tight coils while wet from said aqueous solution whereby the formation of speckled rust on the surfaces of the steel within the laps of the coils is prevented. 